Role of brain function monitoring in the critical care and perioperative settings

The association of bispectral index values and metrics of cerebral perfusion during cardiopulmonary bypass

STUDY OBJECTIVE

Low bispectral index (BIS) values have been associated with adverse postoperative outcomes. However, trials of optimizing BIS by titrating anesthetic administration have reported conflicting results. One potential explanation is that cerebral perfusion may also affect BIS, but the extent of this relationship is not clear. Therefore, we examined whether BIS would be associated with cerebral perfusion during cardiopulmonary bypass, when anesthetic concentration was constant.

DESIGN

Observational cohort study.

SETTING

Cardiac operating room.

PATIENTS

Seventy-nine patients with cardiopulmonary bypass surgery were included.

MEASUREMENTS

Continuous BIS, mean arterial blood pressure (MAP), cerebral blood flow velocity (CBFV), and regional cerebral oxygen saturation (rSO₂) were monitored, with analysis during a period of constant anesthetic. Mean flow index (Mx) was calculated as Pearson correlation between MAP and CBFV. The lower limit of autoregulation (LLA) was identified as the MAP value at which Mx increased >0.4 with decreasing blood pressure. Postoperative delirium was assessed using the 3D-Confusion Assessment Method.

RESULTS

Mean BIS was lower during periods of MAP < LLA compared with BIS when MAP>LLA (mean 49.35 ± 10.40 vs. 50.72 ± 10.04, p = 0.002, mean difference = 1.38 [standard error: 0.42]). There was a dose response effect, with the BIS proportionately decreasing as MAP decreased below LLA (β = 0.15, 95% CI for the average slope across all patients 0.07 to 0.23, p < 0.001). In contrast, BIS was relatively unchanged when MAP was above LLA (β = 0.03, 95% CI for the average slope across all patients -0.02 to 0.09, p = 0.22). Additionally, increasing CBFV and rSO₂ were associated with increasing BIS. Patients with postoperative delirium had lower mean BIS and higher percentage of time duration with BIS <45 compared to patients without delirium.

CONCLUSIONS

There was an association of BIS and metrics of cerebral perfusion during a period of constant anesthetic administration, but the absolute magnitude of change in BIS as MAP decreased below the LLA was small.

Evaluating and monitoring analgesia and sedation in the intensive care unit

Management of analgesia and sedation in the intensive care unit requires evaluation and monitoring of key parameters in order to detect and quantify pain and agitation, and to quantify sedation. The routine use of subjective scales for pain, agitation, and sedation promotes more effective management, including patient-focused titration of medications to specific end-points. The need for frequent measurement reflects the dynamic nature of pain, agitation, and sedation, which change constantly in critically ill patients. Further, close monitoring promotes repeated evaluation of response to therapy, thus helping to avoid over-sedation and to eliminate pain and agitation. Pain assessment tools include self-report (often using a numeric pain scale) for communicative patients and pain scales that incorporate observed behaviors and physiologic measures for noncommunicative patients. Some of these tools have undergone validity testing but more work is needed. Sedation-agitation scales can be used to identify and quantify agitation, and to grade the depth of sedation. Some scales incorporate a step-wise assessment of response to increasingly noxious stimuli and a brief assessment of cognition to define levels of consciousness; these tools can often be quickly performed and easily recalled. Many of the sedation-agitation scales have been extensively tested for inter-rater reliability and validated against a variety of parameters. Objective measurement of indicators of consciousness and brain function, such as with processed electroencephalography signals, holds considerable promise, but has not achieved widespread implementation. Further clarification of the roles of these tools, particularly within the context of patient safety, is needed, as is further technology development to eliminate artifacts and investigation to demonstrate added value.